buffet: Support retries for 5xx errors.

5xx should be generally retried a couple of times
as they usaully mean some issues on server-side.

BUG=chromium:420580
TEST=cros_workon_make buffet --test && manual testing.

Change-Id: Iada5961a77acd203ce55ba5ec914e382f2681df0
Reviewed-on: https://chromium-review.googlesource.com/223310
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Tested-by: Anton Muhin <antonm@chromium.org>
Commit-Queue: Anton Muhin <antonm@chromium.org>
1 file changed